The muscle glycogen debranching enzyme system (amylo-1,6- glycosidase/oligo-1,4 yields 1,4-glucantransferase) consists of two enzymatic activities which have different modes of action; one is a specific type of simple alpha-glycosidase, the other is strictly a disproportionating transferase. The debranching enzyme apparently exists along with phosphorylase and its activating enzyme system as a component of a multi-enzyme complex in heart and skeletal muscle which is designed for the metabolism of glycogen and the regulation of muscle contraction. Study of the role of the glucosidase-transferase as a concomitant part of this protein-glycogen complex associated with the sarcoplasmic reticulum will lead to a clearer understanding of the relationship between the function of the multi-enzyme complex and its structural role in the cell. A study of the relationship between the two debrancher activities, associated with a single polypeptide molecule, will lead to a better understanding of the principles of enzyme structure and function, as well as to a better understanding of the nature of the component structure of the glycogenolytic complex. This will provide a framework to decipher aspects of the mechanism of control that is exerted on the entire system. The use of substrate-like inhibitors has allowed a description of the glucosidase active site and indicated it can serve as a model system to investigate the role and specificity of metabolites of polyamines such as spermine and spermidine, which may act as second messengers of insulin and/or function as catecholamine antagonists. Since these hormones play an important role in the regulation of both heart and skeletal muscle and the multi-enzyme complex appears to be the same in both, this study could lead to a more fundamental understanding of their regulation as well as to a better insight into the relative differences in their control mechanisms.